The present invention relates to a copper-based sliding material whose fatigue resistance and anti-seizure property are improved, a method of manufacturing this copper-based sliding material, and a sliding bearing.
There is a bearing in which Kelmet is used as a copper-based sliding material. A Kelmet bearing in which a copper-and-lead-based sintered alloy is bonded onto a back metal made of steel and in which an overlay is bonded onto the copper-and-lead-base sintered alloy is used as a sliding bearing for an automobile engine, etc. In this Kelmet bearing, the lead (Pb) contained in the copper-and-lead-based sintered alloy, which is an underlayer, is present on a sliding surface even when the overlay is worn and, therefore, the anti-seizure property of the Kelmet bearing is good.
In conventional copper-based sliding materials represented by the Kelmet bearing, the anti-seizure property is thus improved by adding lead (about 20 mass %). However, because lead has an adverse effect on the environment, it is preferred that this element be contained as little as possible or be not used.
Furthermore, in recent automobile engines there are adopted high-rotation design and high-output design, and the temperature and specific load of bearings tend to become high.
In the conventional Kelmet bearing, however, the content of lead that is soft and that has a low melting point is as high as about 20 mass % and, therefore, the conventional Kelmet bearing has low strength and there is a problem in fatigue resistance specially at high temperatures and under high specific loads.
As disclosed by the applicant in JP-A-2000-81523, the addition of hard particles is effective in improving wear resistance.
However, since bismuth (Bi) is used in the invention of JP-A-2000-81523, the material disclosed in the application is inferior in load carrying capacity. Further, in a case where in the material the grain size of hard particles is large, a load acting on one hard particle increases and a shearing force generated at the boundary with the matrix increases, with the result that fatigue is apt to occur from this boundary, having an adverse effect on the fatigue resistance thereof. In addition, hard particles of large size are apt to fall off easily, and in a case where they fall off, the bismuth that is prevented by the presence of hard particles from being carried away becomes apt to be carried away from a sliding surface and the embeddability brought about by bismuth is lost. This posed the problem that a bearing and its counterpart are damaged, and this causes such unsolved problems as the wear resistance and anti-seizure property are deteriorated by abrasive wear.
The invention was completed in taking the above situation into consideration. Therefore, the object of the invention is to provide a copper-based sliding material that ensures a good anti-seizure property even without the use of lead, high fatigue resistance even at high temperatures and under high specific loads, and high wear resistance even under conditions close to those of boundary lubrication, a method of manufacturing the copper-based sliding material, and a sliding bearing made of the copper-based sliding material.
The inventors have found that, in a case where a copper-based sliding material has a structure in which a soft phase and a hard phase are mixed with each other, its anti-seizure property and fatigue resistance are improved.
In other words, on the surface of a sliding material whose structure has a soft phase and a hard phase both mixed with each other, the hard phase becomes convex with the soft phase becoming concave under load or due to sliding wear. Because a lubricating oil is held in this concave portion, the anti-seizure property is improved. The inventors have also found that a concave-convex relationship capable of acting as the reservoir of a lubricating oil is brought about between the soft phase and the hard phase under the condition of (H2/H1)xe2x89xa71.2 where H1 represents the Vickers hardness of the soft phase and H2 represents the Vickers hardness of the hard phase.
Furthermore, because of the presence of the soft and hard phases mixed with each other in the surface portion of the sliding material, the soft phase provides conformability and the hard phase receives loads. Usually, a load received by the hard phase serves as a shearing force acting on the boundary between the hard phase and the soft phase. However, when the hard phase and the soft phase are composed of copper or a copper alloy, the wettability of the phases becomes good, so that a clear boundary between the two phases comes to disappear. Therefore, a load received by the hard phase is distributed in a region defined between the hard phase and the soft phase, resulting in improved fatigue resistance.
Thus, according to the first aspect of the invention, the copper-based sliding material is provided with a structure in which both of a soft phase and a hard phase are mixed with each other, the Vickers hardness of the hard phase being not less than 1.2 times that of the soft phase, so that an excellent anti-seizure property can be obtained even without containing lead. Furthermore, the conformability of the sliding material of the invention is good and the fatigue resistance thereof is high.
In addition, the hard phase that becomes convex in shape and receives the load of a counterpart member contains hard particles. In general, since hard particles improve wear resistance and, at the same time, have the functions of smoothing a counterpart shaft and of removing the bearing alloy components that have adhered, the hard particles improve the anti-seizure property. Moreover, microscopically observed, the load received by the hard phase is received by the hard particles included in the hard phase. The size of the hard particles is as small as 0.1 to 10 xcexcm. Thus, when the content of hard particles is the same, the number of hard particles is large, so that a load acting on each hard particle becomes small, which makes a shearing force acting between hard particles and the matrix small and which makes the surface area of the hard particles large, whereby the hard particles are less apt to fall off and the fatigue resistance is improved. When the content of the hard particles is less than 0.1 vol. %, the hard particles are ineffective in improving the anti-seizure property, wear resistance and fatigue resistance. On the other hand, at a content exceeding 10 vol. %, the attacking property of the sliding material to the counterpart member by the hard particles increases and the anti-seizure property is deteriorated.
According to the second aspect of the invention, the hard particles are ones of at least one kind from the group consisting of borides of metal, silicides of metal, oxides of metal, nitrides of metal, carbides of metal and intermetallic compounds.
According to the third aspect of the invention, the copper alloy phase has a composition of not more than 15 mass % Sn; at least one kind not less than 40 mass % in total selected from the group consisting of Ni, P, Zn, Al, Co, Mn, Ag and Fe; and the balance of Cu and incidental impurities. The reasons therefor is described below.
(a) Not more than 15 mass % Sn
Sn strengthens the matrix and improves fatigue resistance. Furthermore, the addition Sn improves the anti-seizure property of a copper alloy. At a content exceeding 15 mass % Cu, large amounts of Cu-Sn-based intermetallic compounds are generated, resulting in embrittlement.
(b) At least one kind not more than 40 mass % in total selected from the group consisting of Ni, P, Zn, Al, Co, Mn, Ag and Fe.
Each of these elements strengthens the matrix and improves fatigue resistance. Especially, Mn, Co, Ni and Fe are basically present in the hard copper alloy phase because the migration of atoms is less apt to occur at sintering temperatures of 800 to 920xc2x0 C. and, therefore, each of Mn, Co, Ni and Fe acts favorably in a case where a difference in hardness between the hard copper alloy phase and the soft copper phase is to be obtained. When the content of these elements exceeds 40 mass % in total, the hardness of the whole copper-based sintered alloy becomes too high and this is undesirable for a bearing material.
In this case, unless a clear hardness difference between the hard copper alloy phase and the soft copper alloy phase is obtained, the hardness of the hard copper alloy phase does not become not less than 1.2 times that of the soft copper alloy phase in terms of Vickers hardness. For this purpose, it is preferred that elements for strengthening the copper alloy phase (strengthening elements) do not diffuse into the soft phase. However, if strengthening elements do not at all diffuse into the soft phase, the boundary between the hard copper alloy phase and the soft copper alloy phase becomes clear and this is unfavorable in terms of fatigue resistance.
Regarding this respect, the means explained below is adopted according to the fourth aspect of the invention. Namely, after the mixing of at least two kinds of copper alloy powder or the mixing of pure copper powder with at least one kind of copper alloy powder, a mixture is sintered. Therefore, because the at least one kind of powder contains elements for increasing hardness (Ni, P, Zn, Al, Co, Mn, Ag and Fe), the hard phase and soft phase can be easily formed, and the prescribed hardness difference between the two phases can be easily obtained. In addition, because both the soft phase and the hard phase are respectively made of-pure copper and a copper alloy to thereby have good wettability, a part of each of the strengthening elements included in the hard phase diffuses into the soft phase, making the boundary between the two phases unclear.
According to the fifth aspect of the invention, a heat treatment at a temperature of 200 to 500xc2x0 C. is performed for 1 to 20 hours after the final step of sintering, so that the hardness is further increased because of age hardening, for example, in a Cuxe2x80x94Snxe2x80x94Ni alloy. However, because no age hardening occurs in a Cuxe2x80x94Sn alloy, it is easy to obtain the hardness difference between the soft copper alloy phase and the hard copper alloy. Moreover, even in a case where the hard copper alloy phase is made of compositions that do not bring about age hardening, only the soft copper alloy phase can be softened by performing heat treatment after the final rolling by use of rolls when both of the hard and soft copper alloy phases are combined, because the softening temperature of each alloy phase after work hardening differs in dependence on the composition. This also enable the hardness difference to be further increased.
According to the sixth aspect of the invention, a bearing is formed by bonding the above copper-based sliding material onto a back metal, so that the bearing can be used as a sliding bearing of a high-rotation and high-output automobile engine.
According to the seventh aspect of the invention, an overlay comprising a metal and/or a resin is provided on the surface of the copper-based sliding material, so that it is possible to obtain a sliding bearing that is excellent in initial conformability, embeddability of foreign matter and the anti-seizure property.